Civil Engineering Reference
In-Depth Information
shaft (powered by the propulsion unit) that carries out-of-balance weights. Tests have shown that the best
results on both heavy clays and granular soils are obtained when the frequency of vibration is in the range
2200-2400 cycles per minute. Vibration is obviously more effective in granular soil but the effect of vibra-
tion on a 200 mm layer of cohesive soil can effectively double the compactive effort.
A vibrating roller operating without vibration is regarded as being a smooth wheeled roller.
Pneumatic-tyred roller
In its usual form the pneumatic-tyred roller is a container or platform mounted between two axles, the
rear axle generally having three wheels and the forward axle two (so arranged that they track in with the
rear wheels), although some models have five wheels at the back and four at the front. The dead load is
supplied by weights placed in the container to give a mass per wheel range from 1000 kg to over 12000 kg.
The mass per wheel is simply the total mass of the unit divided by the number of wheels. A certain amount
of vertical movement of the wheels is provided for so that the roller can exert a steady pressure on uneven
ground - a useful facility in the initial stages of a fill.
This type of roller originated as a towed unit but is now widely available as a self-propelled vehicle. It
is suitable for most types of soil and has particular advantages on wet cohesive materials.
Sheepsfoot roller
This roller consists of a hollow steel drum from which the feet project, dead weight being provided by
placing water or wet sand inside the drum. It is generally used as a towed assembly (although self-
propelled units are available), with the drums mounted either singly or in pairs.
The feet are usually either club-shaped (100
×
75 mm) or tapered (57
×
57 mm), the number on a 5000 kg
roller varying between 64 and 88. Variations in the shape of the feet have been tried in the USA with a
view to increasing the operating speed.
The sheepsfoot roller is only satisfactory on cohesive soils, but at low moisture contents the resulting
compaction of such soils is probably better than can be obtained with other forms of plant. Their use in
the UK is rather infrequent because of the generally wet conditions.
Deep impact rollers
Recent advances in the development of compaction equipment has seen the introduction of rollers that
can compact significant depths of soil in a single pass. High weight polygonal drum rollers can achieve
much greater compaction efficiency and depths than equivalent weight smooth wheeled rollers. Weighing
around 32000 kg these machines can be used to compact large extents of subgrade on highway construc-
tion projects in about half the time of alternative dead weight rollers.
The grid roller
This is a towed unit consisting of rolls made up from 38 mm diameter steel bars at 130 mm centres, giving
spaces of 90 mm square. The usual mass of the roller is about 5500 kg which can be increased to around
11000 kg by the addition of dead weights; there are generally two rolls, but a third can be added to give
greater coverage. The grid roller is suitable for many soil types, but wet clays tend to adhere to the grid
and convert it into a form of smooth roller.
Rammers and vibrators
Manually controlled power rammers can be used for all soil types and are useful when rolling is impractical
due to restricted site conditions. Vibrating plates produce high dry densities at low moisture content in
sand and gravels and are particularly useful when other plant cannot be used.
